Geopolymer is a 3-D structured alumino-silicate material similar to zeolite, which has a closed framework structure formed mainly by dissolving silicon and aluminum elements out from a mineral powder in a basic solution, and then bonding silica and aluminatetrahedrons to each other, and has excellent fire tolerance, heat insulation, acid and alkaline resistance, and mechanical properties. The basic raw material for the geopolymer is readily available, the preparation process and equipment are simple, and the geopolymer may be prepared at a normal temperature, thus receiving great attentions from various industries, and having a quite high potential for being developed into a new generation of environmentally friendly and green materials.
In 1940s, furnace slag in combination with an alkaline metal such as sodium hydroxide and potassium hydroxide was added by Purdon to cement for solidification and it was found during the research that the addition of alkaline metal can accelerate the hardening of the product (Purdon, A. O., The action of alkalis on blast furnace slag. Journal of the Society of Chemical Industry, 1940. 59: p. 191-202). As a result, the alkaline metal activated furnace slag cement (Trief cement) was largely used in 1950s. Hydrosodalite is prepared at 100° C. by Borchert by reacting kaolinite with sodium hydroxide (Borchert, W. and J. Keidel, Beitrá{umlaut over (g)}ezurReaktionsfähigkeit der Silikatebeiniedrigen Temperaturen-I. Mitteilung. Reaktionenzwischen Kaolin and NaOH. Heidelberger BeiträgezurMineralogie and Petrographie, 1949. 1: p. 2.16.). It is found by Glukovskyin the study on activation of furnace slag with an alkaline metal that the product from the reaction of rock or clay with an alkaline metal contains calcium silicate hydrates and sodium alumino-silicate hydrates, that is, zeolite (Bedard, R. L., S. T. Wilson, L. D. Vail, J. M. Bennett, E. M. Flanigen, n. In Zeolites, n. Figures, and R. A. van Santen, Elsevier: Amsterdam, 1989. 1: p. 375.). In 1963, a type A zeolite was successfully synthesized by Howell by using metahalloysite as a raw material in place of the kaolinite to avoid the production of hydrosodalite. It can be known from above that since 1930s, the development of alumino-silicate materials is very fast. In 1972, a novel product having a compressive strength of up to 15 MPa was synthesized by the French scientist Davidovits by mixing kaolinite and quartz at a weight ratio of 1:1 with a specific concentration of a sodium hydroxide solution, and then follow-up studies was published in 1979; and the product was referred to as “geopolymer” (Davidovits, J., Geopolymers. Journal of Thermal Analysis and calorimetry, 1991. 37(8): p. 1633-1656). Hereto, a series of relevant researches and developments are carried out.
After over four decades of development, it is found after initial intense research that the geopolymer material similar to zeolite in structure has fire protection, thermal shock resistance, corrosion resistance, and other properties, then new use of geopolymer is persistently developed and the physical and chemical properties of the geopolymer are reinforced through various researches. The geopolymer is currently used in various industries in practice, for example, fire-resisting paint, fire compartment in an aircraft, and early setting cement.
A. Geopolymer
The geopolymer is generally produced by mixing, for example, an alumino-silicate mineral, an alkaline metal solution (KOH or NaOH) and a sodium silicate solution. A geopolymer precursor is formed by dissolving a silicon and aluminum rich mineral in a basic solution to evolve out silicon and aluminum elements in ion state on the surface of the mineral particles, and then a geopolymer having a strength is formed after dehydration and drying.
The mechanism of hardening of the geopolymer is not fully understood from the researches up to now, and thus the mechanism of hardening and formation of the geopolymer can only be learned indirectly by from the relation between the additive and the basic solution and the solubility. The solubility of the minerals in the basic solution plays an important role in the formation of the geopolymer, which affects the property of the geopolymer precursor and the geopolymerization. In view of this, it can be known that the final structure of the geopolymer and the properties thereof is closely correlated with the solubility of the minerals. If the solubility of the minerals is poor in the solution, insufficient colloid is produced. In this case, sodium silicate powder or water glass may be added to increase the agglutination between mineral particles and provides silica gel and the element sodium needed in the system, thereby facilitating the geopolymerization between the mineral particles.
B. Principle of Geopolymertechnology
In 1978, anamorphous to semi-crystalline 3-D alumino-silicate material developed by Joseph Davidovits was designated as “geopolymer”, which refers to a mineral polymer formed by geopolymerization. The so-called geopolymerization means that Si and Al colloids are precipitated out on the particle surface when natural mineral materials or waste thereof containing alumino-silicate (in which Al3+ is tetrahedrally or pentahedrally coordinated) and silicate are stood in a reaction environment containing an alkaline activating agent such as high concentration of sodium carbonate or alkaline metal silicate (e.g. K2SiO3 and Na2SiO3), to form a geopolymerprecursor as shown in formula (1). The geopolymerprecursor is further polymerized with the alkaline activating agent, to form the Si—O—Al—O backbone of the geopolymer, as shown in formula (2). The relevant chemical reaction formulas of the geopolymer are shown below:


Generally, the reaction mechanism for forming geopolymer mainly includes (1) dissolution of the alumino-silicate mineral powder in an alkaline activating agent; (2) diffusion of the dissolved silicon and aluminum ions from the surface of the solid particles to the interspace of the particles; (3) polymerization between the alkaline metal silicate solution and the silicon and aluminum ions; and (4) fractional removal of remaining water from the gel, solidification, and hardening to form a geopolymer material of the alumino-silicate. The reaction mechanism is shown in the FIGURE.
C. Conditions for Forming the Geopolymer
The raw materials for the geopolymer may be widely available, and any mineral and waste thereof containing silicon and aluminum elements may be used as long as the additives are well soluble in the basic solution. Therefore, such materials are preferably amorphous. The raw materials for the geopolymer comprise (1) an active filler; (2) an inert filler; and (3) a basic aqueous solution for the geopolymer. A material with the highest performance may be achieved at a suitable mixing ratio if the three ingredients can coordinate with each other.
In addition, considering the environmental issues, the waste thermal insulation materials such as perlite and rock wool containing silicon and aluminum need to be disposed properly. The thermal insulation wools, especially rock wool, are widely used in sound proof, heat preservation, thermal insulation, and fire protection. For example, these thermal insulation wools are widely used as a thermal insulation and heat preservation material in an incinerator or reacting furnace in nuclear power plants of Taiwan Power Company. The thermal insulation and heat preservation material needs to be replaced regularly, to ensure the thermal insulation and heat preservation of the incinerator or reacting furnace. As a result, many waste thermal insulation materials are produced every year, which should be disposed to avoid environmental pollution. The existing technology generally used for disposing the thermal insulation wool includes ultrahigh-pressure compression or plasma smelting, both of which require quantities of energy or special equipment, and thus have difficulties in disposal of a large amount of waste. Moreover, since the thermal insulation wool used in the nuclear power plant may contain radiant substances, the disposal after disuse is very important. Particularly, the disposal of thermal insulation wools after disuse is difficult because a very high temperature is required in the incineration of the thermal insulation wool, causing the incineration to be difficult. Therefore, there is an urgent need for a method for disposing the thermal insulation wool after disuse.
At present, there are a considerably large amount of waste thermal insulation wools to be disposed in Taiwan Power Company. Accordingly, it is advantageous from the perspective of environmental protection to develop an energy-efficient, safe, and inexpensive disposal method.
With the need to seek a method for recycling the waste material after disuse in mind, in the present invention, the waste is found to contain silicon and aluminum ingredients essential to the geopolymer, and an environmentally friendly cement material having economic value is produced by using the waste as a raw material for the geopolymer, thereby accomplishing the present invention.
By means of the environmentally friendly cement and the production method thereof according to the present invention, the waste previously discarded and disposed is recyclablely used as a material having economic value, and the harmful substance and/or radiant substance potentially present in the waste is embedded and solidified in the cement material, thereby solving the problem of waste disposal, which is of great potential from the perspective of environmental protection.